The product was milled and all samples used for test had aparticle size in the range of 40–80 mesh.CTS-g-PAA (without APT) was prepared according tothe same procedure to study the effect of introduced APTon properties of the superabsorbent composite. UncrosslinkedCTS-g-PAA/APT (without MBA) was exhaustivelyextracted with distilled water and ethanol to be free fromhomopolymer in order to investigate the graft polymerizationmechanism.2.4. Measurement of water absorbencySample (0.05 g) was immersed in excess distilled water(500 ml) at room temperature for 8 h to reach swellingequilibrium. Swollen samples were then separated fromunabsorbed water by filtering through a 100-mesh screenunder gravity for 30 min with no blotting of samples.Water absorbency in distilled water of the superabsorbentcomposite, Qeq, was calculated using the followingequation:Qeq ¼m2 m1m1ð1Þwhere m1 and m2 are the weights of the dry sample and theswollen sample, respectively. Qeq is calculated as grams ofwater per gram of sample. Water absorbency of the samplein 0.9 wt% NaCl solution, Qeq’, was tested according to thesame procedure.2.5. CharacterizationIR spectra of samples as KBr pellets were taken using aThermo Nicolet NEXUS TM spectrophotometer. Themicrographs of samples were obtained using ScanningElectron Microscopy (SEM), (JSM-5600LV, JEOL, Ltd.).Before SEM observation, all samples were fixed on aluminumstubs and coated with gold. Thermal stability ofsamples was studied on a Perkin–Elmer TGA-7 thermogravimetricanalyzer (Perkin–Elmer Cetus Instruments,Norwalk, CT), with a temperature range of 25–700 C ata heating rate of 10 C min1 using dry nitrogen purge ata flow rate of 50 ml min1.3. Results and discussion3.1. IR spectraIR spectra of APT, uncrosslinked CTS-g-PAA/APT,CTS, CTS-g-PAA and CTS-g-PAA/APT are shown inFig. 2. As can be seen, intensity of the absorption bandsat 3616 cm1 and 3544 cm1 ascribed to –OH of APTwas decreased in spectrum of uncrosslinked CTS-g-PAA/APT comparing with Fig. 2(a). A series of new absorptionbands at 2928 cm1, 1712 cm1, 1456 cm1 and 1404 cm1ascribed to C–H stretching, –COOH stretching, symmetric–COO stretching and C–H bending appeared in Fig. 2(b).The information from Fig. 2(a) and (b) indicates the participationof –OH group of APT in the graft reaction betweenAPT and AA. As can be seen from Fig. 2(c), the absorptionbands at 1647 cm1, 1598 cm1, 1380 cm1, 1094 cm1 and1037 cm1 are ascribed to C@O of amide I, –NH2, –NHCOof amide III, C3–OH and C6–OH of CTS, respectively.However, the absorption bands of N-H (1598 cm1 and1380 cm1) and C3–OH (1094 cm1) disappeared after thereaction with AA as shown in Fig. 2(d). This informationreveals that –NH2, –NHCO and –OH of CTS took partin graft reaction with AA. The absorption band at1647 cm1 (C@O of amide I) was overlapped by asymmetric–COO stretching and resulted in a broad absorptionband in the range of 1550 cm1 1650 cm1. The newFig. 2. IR spectra of (a) APT, (b) uncrosslinked CTS-g-PAA/APT, (c) CTS, (d) CTS-g-PAA and (e) CTS-g-PAA/APT. Weight ratio of AA to CTS is 7.2;average molecular weight of CTS is 22.9 · 104; MBA content is 2.94 wt%; APT content is 10 wt%; dewatered with methanol.J. Zhang et al. / Carbohydrate Polymers 68 (2007) 367–374 369

absorption bands at 1456 cm1 (C–H), 1405 cm1 (symmetric–COO stretching), 1169 cm1 and 1074 cm1 indicatethe existence of PAA chains. After incorporating APTinto the polymeric network, intensity of absorption band at1576 cm1 ascribed to asymmetric –COO stretchingincreased, which indicates that the chemical environmentof –COO has changed, which may have some influenceon the absorbing ability of the corresponding superabsorbentcomposite. Absorption bands of APT at 1030 cm1and 988 cm1 ascribed to Si–OH also appeared inFig. 2(e), which shows the existence of APT in the composite.It can be concluded from Fig. 2 that graft reaction hastaken place among AA, APT and CTS.3.2. Thermal stabilityThe effect of introduced APT on thermal stability ofCTS-g-PAA was investigated by TGA in this section.TGA curves of CTS-g-PAA and CTS-g-PAA/APT incorporatedwith 10 wt% APT were shown in Fig. 3. As canbe seen from Fig. 3, both CTS-g-PAA and CTS-g-PAA/APT exhibit a three-stage thermal decomposition process.As the temperature increased to 381.7 C, the weight ofsamples decreased gradually implying a loss of moisture,dehydration of saccharide rings and breaking of C–O–Cglycosidic bonds in the main chain of CTS (Douglas &Sergio, 2004). There is a sharp weight loss with increasingtemperature from 381.7 to 391.9 C and 21% of samplewas lost in this temperature range. There was no obviousdifference between CTS-g-PAA and CTS-g-PAA/APT asthe temperature was below 391.9 C. With further increasingtemperature to 501.6 C, CTS-g-PAA exhibits a secondstep decomposition implying the decomposition of carboxylgroups of PAA chains. Similar thermal behavior hasbeen reported by Chen et al. for carboxymethylchitosang-poly(acrylic acid) (Chen & Tan, 2006). During this period,the onset of CTS-g-PAA was at 470.9 C, however, theonset of CTS-g-PAA/APT was not obvious. The sharpweight losses of CTS-g-PAA and CTS-g-PAA/APT at578.4 and 604.3 C, respectively, are suggested to be dueto the thermal decomposition of the PAA chain backbone.As can be seen, CTS-g-PAA/APT showed a lowerweight loss rate and smaller total weight loss within thetemperature of 391.9–700 C comparing with CTS-g-PAA. This result indicates that the incorporation of APTis helpful for the improvement of thermal stability ofCTS-g-PAA. The role of APT in the polymeric networkmay be the main reason for the difference in TGA curves.APT acts as heat barrier, thus delaying the diffusion of volatilethermo-oxidation products to gas, and gas to the composite.This enhances thermal stability of the system.Similar effect of clay on thermal stability of compositematerials has been reported previously (Ray & Okamoto,2003).3.3. Morphological analysisSEM micrographs of APT, CTS-g-PAA and CTS-g-PAA/APT superabsorbent composite were observed andare shown in Fig. 4. As can be seen, APT shows a fibroussurface. CTS-g-PAA shows a tight surface, however, theintroduction of APT forms a relatively loose and fibroussurface. This surface morphology change by introducingAPT may influence the penetration of water into the polymericnetwork, and then may has some influence on swellingability of corresponding superabsorbent composites.3.4. Effect of average molecular weight of CTS on waterabsorbencyMany previously reports from the literature focused onthe effects of external factors, such as initiator, monomerconcentration and ratio of CTS to monomer, on waterabsorbency and graft polymerization between CTS andmonomers (Chen & Tan, 2006; Ge, Pang, & Luo, 2006;Huang, Jin, Li, & Fang, 2006). No information about theeffect of average molecular weight of CTS on water absorbencycan be seen, to the best of our knowledge. The effectof this factor was investigated in this section and shown inFig. 5. Water absorbency of CTS-g-PAA/APT in distilledwater and in 0.9 wt% NaCl solution increased evidentlywith decreasing average molecular weight of CTS. Thismay be attributed to the fact that CTS solution of smalleraverage molecular weight has lower viscosity, which wouldfacilitate the penetration of AA to CTS and enhance graftefficiency, and then the improvement of water absorbency.CTS of higher average molecular weight could restrict thegraft reaction by factors such as steric hindrance, and thenthe decrease of water absorbency.3.5. Effect of MBA content on water absorbencyAccording to Flory’s network theory (Flory, 1953),crosslinking density is a key factor influencing water absorbencyof superabsorbents and water absorbency is in inverseproportion to crosslinking density. Water absorbency can150 300 450 600 75020406080100CTS-g-PAA/APTCTS-g-PAAWeight ( )Temperature (oC)Fig. 3. TGA curves of CTS-g-PAA and CTS-g-PAA/APT. Weight ratioof AA to CTS is 7.2; average molecular weight of CTS is 22.9 · 104; MBAcontent is 2.94 wt%; APT content is 10 wt%; dewatered with methanol.370 J. Zhang et al. / Carbohydrate Polymers 68 (2007) 367–374